(A) Field of the Invention
This invention relates to speed controllers for electric motors and especially relates to speed controllers for electric motors, particularly series-field motors, used to drive battery powered electric vehicles.
(B) History of the Prior Art
In the prior art, motors, especially DC electric motors, were generally controlled by the use of an adjustable large current series resistance. This method of controlling the speed of an electric motor, particularly those used to drive electric vehicles, has been generally unsatisfactory because torque and speed are almost completely interdependent. The interdependency results in very poor speed regulation. In addition, such resistance controllers are extemely inefficient due to high dissipation of electrical energy through the controller resistance elements. This energy dissipation markedly reduces battery life and the allowable time between charges and also increases the minimum acceptable initial size, weight and cost of the battery.
Furthermore, such controllers did not result in fully proportional speed control since frequently the controllers had to be almost in the fully on position before sufficient torque could be developed to start an electric vehicle from a stopped position. The controller would then have to be backed off to reduce the torque after motion started. Such controllers therefore resulted in inconsistent non-uniform motor operation giving a "jerky" feel to the movement of a vehicle.
More recently, an attempt has been made to control the speed of an electric motor by placing a power transistor as an electronic switch between the battery and the motor and activating the transistor by supplying pulses of current to the transistor base. In general, these pulses were of constant duration and the speed was controlled in only a few stages by chopping a current at only a few different speeds. The resulting motor operation is non-uniform, and electrical energy is wasted since current is supplied only at a few levels.